1. Field of the Invention
This invention relates to coatings, and more particularly, to formulations of trivinyl ether monomers useful in forming protective coatings by cationic curing.
Radiation curable coatings and films such as those formed from an acrylate oligomer are in great demand because of their rapid curing properties. However, these compounds are normally highly viscous liquids or solids and thus are unsuitable as diluents for other polymeric components of a radiation curable formulation. Indeed, such compounds themselves require the incorporation of a diluent or solvent for uniform substrate coating, control of coating thickness and curing at low temperatures. Accordingly, low viscosity monofunctional diluents are usually included in their formulations. While these diluents are reactive, they materially reduce the cross-linked density of the finished product and consequently lower abrasion resistance and ability to withstand chemical attack.
Although solvents have been used to reduce viscosity, they are detrimental in radiation curing due to their volatility which presents problems for uniform composition control unless their evaporation prior to radiant exposure is effected. Obviously, such procedure extends processing time and may pose environmental drawbacks.
To some extent, the drawbacks of high viscosity monomers can be reduced by curing at elevated temperatures. However, this alternative significantly adds to the cost of the overall operation in the expenditure of energy, temperature control and loss of more volatile components in the composition or blistering of the coating resulting from entrained volatiles.
Since acrylates are not conducive to cationically induced radiation curing, they require more costly free radical systems which are oxygen inhibited unless effected in an inert atmosphere, generally under a blanket of nitrogen. Although formulation with a photoinitiator which undergoes bimolecular reaction with a hydrogen donor minimizes the inhibitory effect of air, this benefit is realized at the expense of a greatly reduced cure rate. Also, it is found that polymerization or curing in free radical systems ceases almost immediately upon removal from the source of radiation; thus, the cured product usually contains significant amounts of unpolymerized components. Accordingly, it is an aim of research to develop a monomer having the beneficial properties of acrylates but which is amenable to radiation curing at a rapid rate by cationically induced polymerization which is not oxygen inhibited and which permits continued polymerization after removal from the source of radiation exposure.
Finally, it is noted that the unsubstituted acrylates are sensitizers and skin irritants as well as being carcinogenic, so that specialized safety precautions must be taken to protect operators from exposure. Although alkoxylation has lessened irritancy of the acrylates, their carcinogenic properties are not reduced.
Certain epoxy compounds also have been used as coatings; however these monomers and their oligomers are not readily curable by radiation. Thermal curing is generally required to increase the rate of polymerization. While vinyl ethers such as those ethers the polyethylene glycols are curable by UV light, their resulting coatings lack toughness and substrate substantivity. Also these materials have low viscosity such that it is difficult to obtain film build-up prior to curing.
Vinyl ether compounds, and methods for effectively cationically polymerizing such compounds, have been described in the literature. See, for example, U.S. Pat. Nos. 4,161,405; 4,518,788; 4,751,273; 4,775,732; and 4,749,807. However, the art is limited with respect to multifunctional vinyl ether monomers, and to formulations thereof, for forming protective coatings.
Accordingly, it is an object of this invention to provide new protective coatings from multifunctional vinyl ether monomers.
Another object of this invention is to provide substrates coated with a chemically resistant film of cationically cured trivinyl ethers.
Another object is to provide a non-toxic cross linkable copolymeric resin composition which is suitably cured as a film or as a coating on a substrate which possesses toughness, good adhesion, abrasion resistance and resistance to chemical attack in acid or basic media.
Still another object is to provide a more economical process for cross-linking monomeric or polymeric vinyl or epoxy ethers which can be effected in the presence of air at ambient temperature and pressure.
Another object is to provide a vinyl ether composition which is curable at a rapid rate by cationically induced radiation.
These and other objects will become apparent from the following description and disclosure.